1. Field of the Invention
The present invention relates to a method of forming a thin film capacitor having a tantalum oxide dielectric and a manganese dioxide layer.
2. Description of the Prior Art
In the manufacture of thin film capacitors, it has heretofore been necessary to first form a thin film of tantalum as the first electrode on a dielectric substrate. This composite would then be placed in an anodizing bath to form a layer of tantalum oxide on the tantalum film, which procedure required an abnormally long period of time, thereby adding greatly to the cost of manufacturing a capacitor. Capacitors produced by such methods required the use of tantalum metal, which itself added greatly to the cost thereof. Further, when in the past a definite capacitor pattern was required, complicated and time consuming photolithographic, or like techniques were required to produce well defined edges to reduce edge effect problems.
Prior art efforts to form tantalum oxide dielectric layers on non-tantalum electrodes resulted in a significant change in the value of breakdown voltage with the resultant reduction of capacitance and the increase in electrical series resistance.
It has also been known in the prior art to dispose a film or layer of manganese dioxide over the tantalum oxide dielectric to improve both the working voltage as well as the yield of relatively large area capacitors. However, two severe problems have been encountered with the use of manganese dioxide films over the tantalum oxide dielectric film. First, the loss tangent of the capacitors with manganese dioxide layers is substantially higher than conventional tantalum oxide film capacitors. Secondly, the temperature coefficient of capacitance is significantly increased with the use of a manganese dioxide layer. It is generally believed that the first problem and to some extent the second problem are due to the high resistivity of the manganese dioxide layer. Although manganese dioxide layers or films can be prepared with resistivities lower than 0.1 ohm-cm., deposition methods that are compatible with tantalum oxide film capacitor fabrication, that is, methods that do not cause irreparable damage to tantalum oxide films, generally yield manganese dioxide films with high resistivities of about 1000 ohm-cm.